The invention relates to a control system with actuators which can be separately energized and deenergized.
An advantageous control system of this type has already been described in a published German Patent Application No. P 25 10 992.6.
The individual actuators are each associated with a partial manipulated variable which is a part of an over-all manipulated variable. The control system or controller has a two-position control mode and comprises a conventional three-position controller with an output signal supplied to a stepping switch energizing the actuators. The stepping switch unit comprises a constant current circuit for each output of the three-position control unit, followed by a capacitor-/amplifier arrangement containing an operational amplifier with a high input resistance and an output connected with the parallel circuit of threshold switches with a stepped reference voltage, wherein one of these switches acts on a single one of the actuators.
In one form of the above control system the stepping switch unit is coupled with a two-position control unit by means of one AND-/OR-gate combination provided for each of the threshold switches, where the two-position control unit has an hysteresis (H.sub.Z) lying in the non-operation region between the hysteresis curves of the three-position control unit. This coupling of the stepping switch unit is effected in such a manner, that the inputs of the AND-gates are each connected through an analog-TTL-converter with the outputs of the associated threshold switch on one hand and with the outputs of the associated OR-gates on the other hand, while the inputs of the OR-gates on one hand are jointly connected through and analog-TTL-converter with the output of the two-position control unit on the other hand are connected with the threshold switch of the succeeding switching stage. By means of this control system on one hand a control of the speed of switching on and switching off of the actuators is ensured and is proportional to the control error independently of the kind of the disturbance variable, and in addition a corrective overshoot of the three-position control is achieved, so that after the set-point conditions are reached, set point deviations which lie within the narrow limits of the hysteresis curve of the two-point control unit are already readjusted immediately.
The present invention has the objective to prevent the overshoot or undershoot of the set-point value or reference value, which occurs in the presence of large directed control errors due to the large change of the manipulated variable caused by this, for example in a heating system the removal of the nightly load drop.
In accordance with the invention, the pulse interrupters of the three-position control unit are designed as switching elements and a NOR-gate is provided between the output of the two-position control unit and the logic circuit. The other input of the NOR-gate is connected with the switching elements of the three-position control unit through an inverter-threshold switch and a double inverter (TTL-converter) formed by interconnected npn- and pnp-transistors in such a manner that the base of the pnp-transistor is connected with the switching element of the output for positive control errors and the basis of the npn-transistor is connected with the switching element of the output for negative control errors for the three-position control unit.
The invention utilizes the property of a three-position controller with differentiating feedback, where for a close approach to the actual value in the controlled system to the set-point or reference value, pulses with a direction opposed to the prior pulse direction are generated even before the set-point value is reached. In the above known control system the de-energizing of the last energized actuator is dependent on a discharging of the capacitor of the constant current source down to a threshold level which depends on the associated threshold switch. With the control system according to the invention in contrast to this known system the first occurence of these opposed pulses immediately results in a switching off of the last energized actuator. It has been found, that by means of this control action possible with the system according to the invention the unwanted overshooting of the set-point or reference value is substantially completely prevented, even under extreme operating conditions.